Feline immunodeficiency virus (FIV) shares many features with the primate lentiviruses and is thus considered a valuable experimental model to study AIDS intervention therapies. As with HIV-1, FIV entry is a multistep process. FIV envelope (Env) must sequentially engage CD134 and CXCR4, triggering conformational changes in Env that ultimately lead to fusion between the viral and host cell membranes. Beside its critical role in viral entry, FIV Env is also the primary target for antibody-mediated neutralization. However, antibodies elicited naturally (or experimentally) against FIV Env either failed or weakly neutralize virus infection. Our recent studies on the mechanism of FIV neutralization indicate that monoclonal antibodies (MAb) elicited against the V3 region, the major immunodominant domain of FIV Env, are CD134 induced (CD134i) antibodies. They blocked Env-CXCR4 interaction, but weakly neutralized infection. However, neutralization was greatly enhanced in the presence of soluble CD134. We hypothezise that the CD134i epitopes are only transiently exposed after Env binds CD134 but before CXCR4 interaction. Kinetic and/or steric factors then restrict the access of the antibodies to the V3 region, and hence their efficacy at neutralization. The central goal of the proposed work is to develop CD134-based molecules capable to render determinants on the V3 region accessible to neutralizing and blocking agents. The specific aims are to: 1. develop and test in vitro chimeric proteins containing a CD134 moiety attached via a flexible polypeptide linker to a second moiety consisting of the single-chain variable region (scFv) of our most potent anti-V3 MAbs. The concept is that binding of the CD134 moiety to FIV Env will induce the exposure of the anti-V3 MAb epitope; the scFv moiety would then bind, thereby blocking Env-CXCR4 interaction. 2. extend the model proposed in aim 1 by replacing the scFv moiety by the extracellular loops of CXCR4. Although the second ECL is critical for FIV infection, all ECL including the N-terminal tail will be tested. [unreadable] [unreadable] [unreadable]